Restore focus if another modal dialog is opened on top of FileDialog

An example of this would be the FFmpeg custom options dialog.  When
the options dialog would close, focus would be lost and with no
apparent way to get it back when using the keyboard.

This change tracks the modal dialogs that are opened after the
current file dialog and that are descendants of the current file
dialog.  Once all of the descendants are closed, the current file
dialog will restore the focus and make sure it is the topmost
window.

This only affected Windows.

lib-src/FileDialog/win/FileDialogPrivate.h

@@ -16,6 +16,8 @@
 
 #include <windows.h>
 
+#include <wx/modalhook.h>
+
 //-------------------------------------------------------------------------
 // FileDialog
 //-------------------------------------------------------------------------
@@ -36,9 +38,8 @@ class FileDialog : public FileDialogBase
 
    virtual void GetPaths(wxArrayString& paths) const;
    virtual void GetFilenames(wxArrayString& files) const;
-   void OnSize(wxSizeEvent & e);
    virtual int ShowModal();
-   
+
 protected:
    // -----------------------------------------
    // wxMSW-specific implementation from now on
@@ -98,6 +99,23 @@ private:
 
    wxPanel *mRoot;
 
+   class Disabler : public wxModalDialogHook
+   {
+   public:
+      Disabler();
+      void Init(wxWindow *root, HWND hwnd);
+
+   protected:
+      int Enter(wxDialog *dialog);
+      void Exit(wxDialog *dialog);
+      bool IsChild(const wxDialog *dialog) const;
+
+   private:
+      wxWindow *mRoot;
+      HWND mHwnd;
+      int mModalCount;
+   } mDisabler;
+
    DECLARE_DYNAMIC_CLASS(FileDialog)
    DECLARE_NO_COPY_CLASS(FileDialog)
 };